One hp motor draws 746 watts per hour.
That is incorrect! That would be a 100% efficient motor, and NO SUCH animal exists!
Motors are rated for output HP, so if a motor nameplate says 1 HP, that is OUTPUT, not INPUT. You could easily draw 1500 watts to get an output of 746 watts. It depends on the motor efficiency, load, service factor...
The best way to estimate your operating cost is to take FLA (full load amps) and multiply it by operating voltage, then you get watts.
Amps x Volts = Watts
For example, I looked up a Baldor 1HP single phase motor. It operated at 115 volt, 13 amps. (starting current was 37 Amps, but only for a short time)
13 amps x 115 volts = 1,495 watts (divide by 1000 to get KW)
1.495 kwh x 24 hours = 35.88 kwh/day x $0.11 = $3.95/day x 30 days = 118.50 for 30 days.
This is just a way to estimate electric cost. Other factors play a role in actual cost.
I can easily believe a 5 or 7.5 hp motor would run 300 bucks a month, and the 1/2 hp motor at 35-40 per month.
The best way to know what electricity costs for an existing motor is to measure the amps and volts in actual operating conditions. Then you can find out how efficient you are and see if you can take steps to increase efficiency.
Three phase motors are quite a bit more efficient than single phase motors. There are a number of Variable Frequency and Inverter Drives that will operate three phase motors on single phase power.
